Clays are commercially available and comprise one of the least expensive groups of raw materials. When heated, certain varieties are known to partially vitrify before melting. However, the degree of vitrification is customarily quite small and is dependent upon composition, the crystallinity of the clay, impurities present therein, and particle size. For example, it is known that certain iron-rich montmorillonites partially vitrify at 1050.degree. C. Complete vitrification thereof will take place at 12.degree. C. but is always accompanied with large scale melting, forming, bloat holes, and essentially total deformation. In contrast, other montmorillonites are known wherein high temperature crystalline phases develop at temperatures above 900.degree. C. which may persist up to temperatures in excess of 1300.degree. C. Yet, it will be recognized that low temperature vitrification of clay to a homogeneous glass, i.e., the elimination of melting batches required in the conventional practice of glass manufacture, can result in numerous savings.
Clays are built up of sheet structures involving two units. The first unit consists of Si-O tetrahedra linked together by sharing oxygens so that the bases are coplanar and the apices point in the same direction. The other unit consists of an octahedral sheet in which Mg.sup.+.sup.2, Fe.sup.+.sup.2, Al.sup.+.sup.3, etc., are located in 6-fold coordination. The two units are joined together by sharing oxygens. Unshared oxygen sites are taken by hydroxyls.
The montmorillonites are 3-layer silicates consisting of an octahedral sheet sandwiched between two tetrahedral sheets and conforming to the general formula: EQU Al.sub.2-x (Mg,Fe).sub.1.5x Si.sub.4 O.sub.10 (OH).sub.2
the montmorillonites are theoretically balanced composite layers held together by van der Waal forces. Nevertheless, they nearly always possess Mg.sup.+.sup.2 for Al.sup.+.sup.3 substitutions resulting in up to 0.32 units of charge deficiencies which are compensated by loosely held cations on the surfaces, edges, and in the interlayer positions. These cations are easily replaceable and impart cation exchange capability to the clays.
The appended FIGURE graphically depicts two types of differential thermal analysis (DTA) curves observed with montmorillonite clays. Loss of surface or interlayer water and hydroxyl are indicated by the endothermic peaks at 150.degree.-200.degree. C. and 600.degree.-700.degree. C., respectively. One type of montmorillonite, illustrated by Curve A, demonstrated a sudden rise in slope at about 900.degree. C., whereas Curves B and C, representing the second type of montmorillonite, manifested only a gradual change after dehydroxylization. Those montmorillonites represented by Curve A were quite refractory and vitrified to but a small extent. Hence, the rise in slope is believed to be a result of the nucleation of dense crystalline phases which cause a substantial decrease in vitrification below the melting temperature.
Montmorillonites of the second type could be sintered at a temperature around 900.degree. C. to a porous ceramic body consisting essentially of crystalline montmorillonite-anhydrite with some glass. Further heating to higher temperatures caused the development of such phases as anorthite, cristobalite, mullite, and spinel. The proportion of glass increased with the rise in temperature but remained subordinate to the crystal phases until large scale melting and deformation began. Uncontrolled heating to temperatures in the vicinity of 1100.degree. C. yielded a foamed glassy material. The foaming was considered to be caused by the evolution of steam and CO.sub.2 resulting from dehydroxylization, burning of carbonaceous materials, and the decomposition of carbonates.